Fluoride has become a notable toxicological environmental hazard worldwide because it is often found in groundwater. In the present study, hydroxyapatite adsorbent was synthesized from eggshell waste to remove fluoride from aqueous solution. XRD, FT-IR, and TGA techniques were used to characterize the prepared adsorbent. Batch adsorption studies were performed to examine the adsorption capacity of hydroxyapatite such as the effect of the initial pH of the solution, contact time, adsorbent dose, and initial fluoride concentration. The fluoride ion-selective electrode was used to determine the fluoride removal efficiency. 98.8% of fluoride was removed at pH 3.0, but at pH ~7.0, 85% of fluoride was removed; it shows that the fluoride adsorption is pH dependent. The adsorption isotherm studies (Langmuir and Freundlich models) and the experimental results for the removal of fluoride showed that the Langmuir model was more favorable and the reaction followed pseudo-second-order kinetics. In real water samples, the prepared hydroxyapatite derived from eggshell exhibited 81% removal efficiency. Our results indicate that eggshell waste-derived hydroxyapatite may be an alternative source for defluoridation in developing countries.
Water is an essential element in human life which may be adulterated by industrial wastes and natural causes. It is estimated that poor quality of drinking water causes 80% of diseases worldwide. It has been reported that 65% of endemic fluorosis is caused by fluoride-contaminated drinking water [
Recently, the adsorption method is shown to be effective in the removal of fluoride even at low concentrations, and additionally, it has low maintenance cost [
Among them, the wet chemical precipitation method has shown to be advantageous over other methods due to its cost-effective simple procedure [
Chicken eggshells were collected from local restaurants in Adama city. Analytical-grade chemicals and reagents were purchased from Addis Ababa (NEWAY PLC, Chemicals). All test solutions were prepared with deionized water.
Hydroxyapatite powder was synthesized from eggshell waste and phosphoric acid through wet chemical precipitation in two phases. In phase I, the surface of the eggshells was washed three times with distilled water and the internal thin layer of the shell was removed to decrease the collagens. The cleaned raw shells were boiled in distilled water for 1 hr at 100°C to remove impurities and organic matter. The eggshell was dried in an oven at 80°C for 3 hours in order to crush and grind using a pestle and mortar to obtain a powder. The eggshell residue was sieved with a 150
In phase II, a stoichiometric amount of calcined eggshell powder (calcium oxide) and 0.3 M H3PO4 (to obtain the Ca/P mole ratio equal to 1.67) was added and dispersed in distilled water in a 500 mL beaker. Analytical-grade phosphoric acid was diluted to 0.3 M and added dropwise into the suspension at room temperature by monitoring the pH with a pH meter until the pH reaches 8.5. The solution was subjected to aging treatment for 12 hr at room temperature followed by stirring on a magnetic stirrer for 30 minutes without heating and left for an extra 10 hr for precipitate formation. The precipitate was washed and filtered using a filter (Whitman # 1) paper. Finally, the precipitate obtained was dried in an oven at 80°C for 3 hr and calcined (900°C) in a furnace on a ceramic crucible for 2 hr to obtain the hydroxyapatite (HA) powder. The synthesized HA powder was stored in a dry place and taken for further characterization.
TGA analysis was used (Shimadzu DTG-60 Plus instrument) for the determination of the calcination temperatures. The prepared eggshell and synthesized HA powders were sieved with a 50
The HA samples were grounded using a marble mortar and pestle before distributing 100 mg of the ground sample powder over a 10 mm diameter. The prepared HA samples were analyzed by XRD using a CuK
For the phase identification step, the X-ray diffraction patterns were directly compared to the files to HA from the Joint Committee Powder Diffraction Standards (JCPDS, card no. 09-432) as was supplied by the International Centre for Diffraction Data (ICDD).
Scherrer’s formula was used to determine the crystal size from the XRD pattern. As per this equation, a single crystal dimension (nm) can be calculated from the peak broadening.
In the above equation,
Fourier transform infrared spectroscopy (FT-IR) was handled by a Spectrum 65 FT-IR (PerkinElmer). Sample preparation was done by mixing 2 mg of each sample with 300 mg of potassium bromide (KBr), compressed to form a pellet and then placed on a specimen holder; the spectrum was recorded from 400 to 4000 cm-1.
After characterization of the synthesized HA, four samples of the powder were accurately weighed on an electronic mass balance for the defluoridation test. The samples have constant intervals of 1 g, 3 g, 5 g, and 7 g.
Fluoride stock solution was prepared by dissolving 2.21 g of sodium fluoride in 1000 mL of distilled water in a plastic standard flask. Equal intervals of fluoride solutions of 5, 10, 15, and 20 mg/L fluoride were prepared by serial dilution from the stock solution for the defluoridation test.
Six samples (30 mL) of groundwater (raw water) were donated from Bofo and Serenity sites under the supervision of OSHO Lab Technical for the practical defluoridation test. Three samples from Bofo located at East Shewa, Lome district, and three samples from Serenity located at East Shew around Meki town were carefully measured with the graduated cylinder and kept in a 50 mL plastic beaker until their fluoride concentration was determined. The fluoride concentrations of water samples were 8.3 and 10.5 mg/L, respectively, and both are above the WHO guideline. The defluoridation test was done with 1, 3, and 5 g doses of synthesized HA powder by using the same procedure for optimizations.
TISAB is essential in ion-selective electrode measurements because it masks minor changes made in the ionic strength of the solution and hence increases the accuracy of the reading. 7 g trisodium citrate (Na2C6H2O7), 56 g sodium chloride (NaCl), and 2 g EDTA are dissolved into 500 mL of double-distilled water. After the solution was dissolved, 57 g of glacial acetic acid is added into it, and finally, 5 M sodium hydroxide was added until the pH reached 5.3, then transferred to a 1000 mL volumetric flask, and brought up to the mark using double-distilled water.
20 mL of fluoride solution with different fluoride ion concentrations was prepared in constant intervals, and 2 mL of TISAB was added to each solution. The potential (
A fluoride ion-selective electrode was used to determine the fluoride ion concentration. For this test, the HA powders were separately added on to 30 mL of the known concentration of fluoride water (5, 10, 15, and 20 mg/L) in a 50 mL plastic beaker. To eliminate the interference effect of complex ions in the solution and to maintain ionic strength and the pH, 2 mL of total ionic strength adjustment buffer (TISAB) solution (10 : 1 volumetric ratio) was added. In this way, the effect of various parameters like contact time, pH, adsorbent dose, and initial fluoride concentration was obtained by changing a parameter and keeping the other parameters constant. Finally, the equilibrium fluoride concentration (residual) for each test was determined using a pH meter in combination with a fluoride-selective electrode, and the pH was measured with a pH meter. The electrode was calibrated prior to each experiment.
All experiments were conducted using 30 mL of fluoride solution (10 mg/L) taken in four different 50 mL plastic beakers. 1 g, 3 g, 5 g, and 7 g HA samples were added in this solution for different contact times. Then, the solution was stirred for 1 minute at 25°C to reach equilibrium on a homogenous solution. Each sample was taken at a specified time interval for their contact times 1 hr, 3 hr, 5 hr, 7 hr, 11 hr, and 24 hr and filtered with a Whitman filter paper (no. 1) before analysis. The fluoride-selective electrode reads the electrovolt in terms of a millivolt. The millivolt was converted into mg/L of residual fluoride concentration using predetermined calibration slop by Microsoft Excel. The percentage of adsorption efficiency and the fluoride removal capacity (mg of fluoride ion adsorbed per gram of adsorbent) at a given contact time for the HA adsorbent was calculated using the following:
In the above equation,
In this study, the effect of major parameters like adsorbent dose, initial fluoride concentration, contact time, and pH was optimized to investigate the maximum defluoridation efficiency of the synthesized natural hydroxyapatite (HA) from the eggshell.
Experimental examinations were carried at different concentrations of 1, 3, 5, and 7 g of adsorbent in 10 mg/L of fluoride initial concentration at pH 3.
Residual anionic fluoride concentration was measured at different contact times of adsorption, 1, 3, 5, 7, 11, and 24 hours, with 5 g of adsorbent (HA) to study the effect of contact time. Other parameters like pH, the concentration of the solution, and HA dosage remain constant.
To study the effect of initial fluoride concentration, experiments were conducted at various fluoride concentrations (5, 10, 15, and 20 mg/L) at a constant temperature, pH (pH 3), adsorbent dose (5 g), and contact time (5 hr).
To investigate the effect of pH on defluoridation, the test solutions containing the optimized concentration of fluoride were changed to pH values of 3, 5, 7, and 9 using HCl (1 N) and NaOH (1 N) and the adsorbent mass remained constant through acidic, neutral, and basic media. Then, the determined 5 g HA was added in into each test solution separately and stirred for 1 minute to reach equilibrium. Residual F− ion concentrations were evaluated in each experiment after 5 hr contact time. Finally, pH versus percentage removal graph was plotted to explain the adsorption performance of the adsorbent HA at each pH.
The relationship between the amounts of substances sorbed at a constant temperature and its concentration equilibrium solution is called adsorption isotherm. The frequently used adsorption isotherm models for surface average analysis are Langmuir and Freundlich isotherms.
The Langmuir isotherm model is widely used to quantify the amount adsorbed on the adsorbent as a function of concentration at a given temperature. Theoretically, the adsorbent has a limited number of available sites for the adsorbate. Therefore, beyond saturation value, no further adsorption can occur. The linear form of the Langmuir is as follows:
The Langmuir isotherm can be expressed by a dimensionless separation factor or equilibrium parameter,
The value of
Dimensionless separation factors (
Concentration (mg/L) | 5 | 10 | 15 | 20 |
0.235 | 0.136 | 0.095 | 0.073 |
Kinetic data obtained by varying the contact time using a constant adsorbent dose of 5 g HA,
Time (hr) | Residual fluoride | ||||
---|---|---|---|---|---|
1 | 0.15 | 0.0591 | 16.92047 | -8.1117 | -2.82855 |
3 | 0.12 | 0.0592 | 58.67567 | -8.5171 | -2.8265 |
5 | 0.10 | 0.0594 | 84.17508 | ∞ | -2.8234 |
7 | 0.11 | 0.0593 | 118.04384 | -9.2103 | -2.8251 |
9 | 0.11 | 0.0592 | 151.77065 | -9.2103 | -1.8251 |
11 | 0.13 | 0.0592 | 185.81081 | -8.5171 | -2.8268 |
24 | 0.13 | 0.0592 | 405.40545 | -8.5171 | -2.8268 |
The Freundlich isotherm equation takes into account repulsive interactions between adsorbed solute particles and also accounts for surface heterogeneities. The logarithm form of the Freundlich isotherm is given as follows:
Finally, the applicability of two adsorption isotherms (Langmuir and Freundlich) can be compared by evaluating the multiple regression correlation coefficients,
The isoelectric point or point of zero charges (
Plot of
Initial pH | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
Final pH | 2.90 | 3.60 | 4.50 | 5.40 | 6.30 | 7.21 | 7.90 | 8.30 | 8.90 | 9.86 |
Change in pH | 0.90 | 0.60 | 0.50 | 0.40 | 0.30 | 0.21 | 0 | -0.7 | -1.1 | -1.14 |
It is very important to know the rate at which the adsorption process takes place and the factors that control the rate of the process; for this purpose, the kinetics of the process were evaluated. The adsorption kinetic studies describe the rate of uptake of the adsorbate molecule; in this case, fluoride ion onto adsorbent, the rate depends on the physicochemical characteristics of the adsorbate and adsorbent, pH, temperature, and concentration. To describe the adsorption kinetic behavior, the following models are used.
The pseudo-first-order differential equation is generally expressed as follows [
In the above equation,
Equation (
The rate constant
Adsorption kinetic parameter for pseudo-first-order kinetics.
Pseudo-first-order kinetics | Intercept | |||
-8.634 | 0.005 | 0.0089 | -0.1602 |
The rate of a pseudo-second-order differential equation is given as follows [
The values of
Adsorption kinetic parameter for pseudo-second-order kinetics.
Pseudo-second-order kinetics | Intercept | |||
2.1152 | 134.048 | 0.999 | 0.0596 |
The calcium precursor used for the HA synthesis was derived from the decomposition of eggshell. The complete calcium precursor decomposition temperature was obtained from the TGA/DTA result. The thermogravimetric analysis (TGA) of the eggshell powder was carried out between room temperature and 1000°C in order to determine the thermal stability and the decomposition temperature of eggshell powder. The TGA result revealed that a significant mass loss was observed between the temperatures 600 and 800°C, probably due to the removal of impurities, and the eggshell residue was stable above 800°C as indicated in Figure
TGA/DTA result of eggshell powder.
XRD pattern of calcinated eggshell at 850°C.
TGA/DTA experiments were done to determine the calcination temperature for the formation of pure-phase hydroxyapatite synthesized from calcined eggshell residue and phosphoric acid. As illustrated in Figure
TGA/DTA pattern for the synthesized HA.
XRD pattern of HA powder calcined at 900°C.
FT-IR patterns of calcinated HA powder
The concentration range was selected based on the preliminary test results at room temperature. The experiment was conducted on different masses of adsorbent at 1 g 3 g, 5 g, and 7 g in 10 mg/L of the initial fluoride concentration with 5 hr contact time at pH 3. As described in Figure
Investigation of the optimum dose test using 10 mg/L initial fluoride concentration, pH 3, and contact time of 5 hr.
To determine the effect of contact time on adsorption of fluoride on hydroxyapatite, batch experiments were carried with different contact times (1 hr, 3 hr, 5 hr, 7 hr, 9 hr, and 11 hr) under constant dosage (5 g, 10 g/L fluoride concentration) and pH (pH 3) at room temperature. As shown in Figure
Fluoride removal efficiency as a function of contact time at 5 g of the HA dose,
For this test, the initial concentration of fluoride is selected randomly with 5 increments from 5 to 20 mg/L. The effects of other parameters remain constant, and the adsorbent dose was 5 g with 30 mL solution. The results indicate that the initial fluoride concentration has an influence on the removal capacity of the adsorbent (Figure
The effect of initial fluoride concentration on percentage removal of fluoride using 5 g adsorbent, 5 hr contact time, and
The adsorption process is controlled by the pH of the adsorbate solution. To make inclusive all the 3 media, the pH range was deliberately selected from 3 to 9. The effect of pH was investigated by varying the pH from 3 to 9 with 2 increments. As illustrated in Figure
(a) The effect of pH on the percentage removal of fluoride at 5 g adsorbent and 5 hr. (b) Fluoride removal efficiency of synthesized HA (sHA) and commercial HA on raw water samples from Bofo and Maki (5 g dose, 5 hr contact time, and pH 7).
Samples of water were collected from two different locations of the Oromia region, East Showa district, near Adama city. Sample 1 is from Bofo around the Bati Lome District and sample 2 from around Maki Town; the secured groundwater had fluoride ion concentration of 8.3 mg/L and 10.5 mg/L, respectively, which is still more than the permissible limit of WHO. After adsorption, the fluoride ion concentration of the two samples was measured and it was found that the fluoride ion concentration was decreased significantly in adsorbent doses of 1 g, 3 g, and 5 g, at a temperature of 25°C, rotation speed of 400 rpm for 1 minute steering time, 5 h contact time, and optimum
In this investigation, the data based on the optimized fluoride concentration were interpreted in Table
Langmuir’s adsorption isotherm data for concentration versus adsorption capacity.
5 | 0.25 | 0.0285 | -0.6020 | -1.5451 | 8.7719 |
10 | 0.50 | 0.0570 | -0.3010 | -1.2441 | 8.7719 |
15 | 0.96 | 0.0842 | -0.0177 | -1.0746 | 11.4014 |
20 | 1.6 | 0.1104 | -0.2041 | -0.9570 | 14.4927 |
Linearized Langmuir adsorption isotherm for fluoride adsorption
Freundlich isotherm graph of adsorption fluoride by HA.
Langmuir and Freundlich isotherm model parameters describing the HA fluoride adsorption at a constant adsorbent dose of 5 g, contact time of 5 hr, and
Parameter | Langmuir model | Parameter | Freundlich model |
---|---|---|---|
0.2212 | 0.3879 | ||
0.6349 | 0.914 | ||
0.9657 | 0.763 |
In the present study, the kinetics of defluoridation was carried out to study the behavior of the synthesized hydroxyapatite Table
The different parameters of pseudo-first- and pseudo-second-order kinetics are given in Tables
For kinetic studies, 5 g of HA would be contacted with 30 mL of fluoride solution having a fluoride concentration of 10 g, at a pH of 3, and shaken at 200 rpm and room temperature (Table
Pseudo-first-order kinetic result.
From the result of adsorption kinetic first-order and second-order data, it is possible to summarize the adsorption behavior of the adsorbents prepared (Table
Pseudo-second-order kinetic result.
In the present investigation, pure-phase hydroxyapatite was successfully synthesized from chicken eggshell through the wet precipitation method for defluoridation of water by adsorption. The main advantage of this eco-friendly method is that water is the only by-product. The XRD result revealed that the synthesized HA powder was pure phase with a hexagonal structure. The prepared HA (adsorbent) was applied to the real water samples under ideal conditions and found to be effective with 81% fluoride removal efficiency. Our results show that eggshell waste-derived hydroxyapatite may be an alternative source for defluoridation in developing countries. Additionally, natural raw water contains ions that compact with fluoride for adsorption such as HCO3−, SO42−, and Cl−. The effect of these coions should be investigated so that it helps to selectively use the adsorbent in preference to other ions.
The data used to support the findings of this study are available from the corresponding authors upon request.
The authors declare that they have no conflicts of interest.
The study and the first draft writing were conducted by Kifle Workeneh, and the supervision and edition were done by Prof. Rajalakshmanan Eswaramoorthy. The coauthors Enyew Amare Zereffa and Toshome Abdo Segne participated in analyzing the results and drafting the manuscript.
The authors are grateful to the management of Adama Science and Technology University for providing the financial support towards this research work.